Requirements on measurement equipment for measuring electrical signals, e.g. in terms of frequency bandwidth, are becoming increasingly harder. Digital measurement equipment, such as digital oscilloscopes, digital spectrum analyzers, and the like are often preferred over their analog counterparts, e.g. due to better processing and analysis capabilities. In such digital measurement equipment, analog-to-digital converters (ADCs) are normally utilized to digitize analog signals, thereby forming digital signals.
The signal frequency bandwidth that the digital measurement equipment should be capable of handling may in some applications extend from a few Hz up to several GHz, or even higher. Such relatively hard bandwidth requirements on the digital measurement equipment in turn impose relatively hard bandwidth requirements on analog circuitry in the interfaces between analog input ports and ADCs of the digital measurement equipment. Examples of such analog circuitry include circuitry for converting an analog signal from a single-ended to a differential representation, impedance converters, buffers, and the like. One solution to design such analog circuitry is to use amplifier circuitry including amplifiers with wide-enough bandwidth to cope with the requirements. However, the design and manufacture of such amplifiers is normally far from trivial, which can result in an undesirably high production cost. An alternative solution for providing conversion from single-ended to differential signal representation, and also for providing impedance conversion if necessary, is to use a passive circuit normally referred to as a balun. A balun may be designed to handle relatively high signal frequencies at a comparably lower cost than the above-mentioned amplifier circuitry. However, a balun normally has relatively poor signal transfer properties in a low-frequency band, extending from 0 Hz up to some frequency that depends on the particular type of balun. Hence, covering the whole signal frequency band of interest using a balun in said interface might not be possible.